Coating surfaces



Oct. 14, 1969 s. w. ms, JR., ET 3,472,679

COATING SURFACES Filed Nov. 27, 1968 26 ms "vww INVESTORS SAMUEL w. ms JR. YUEN-SHENG CHIANG BY amp 72 W ATTORNEY United States Patent Int. Cl. C23c 13/02 US. Cl. 117-93 15 Claims ABSTRACT OF THE DISCLOSURE A process for coating a film on a surface which comprises; establishing a carrier gas stream flowing from a source of coating material toward a surface to be coated With said material; said carrier gas stream being made up substantially of a carrier gas and a coating material, exciting said carrier gas into plasma state in the vicinity of said coating material, and diminishing said plasma at a point intermediate said source and said surface to be coated, whereby said carrier stream in a vapor state contacts said surface, and a film of said coating material is formed thereon, with said coating material being selected from the group comprising selenium, arsenic, tellurium, antimony, bismuth, thallium, sulfur, and mixtures thereof.

RELATED APPLICATIONS This application is a continuation-in-part of our copend ing application Ser. No. 482,354, filed on Aug. 25, 1965 now abandoned.

BACKGROUND OF THE INVENTION This invention relates to improved vapor coating techniques plasma transport for producing highly uniform films on support substrates.

According to known vapor coating techniques which can be used, for example, in the production of xerographic plates comprising a layer of an insulating photoconductive material overlying a supporting substrate, a coating material is vaporized in a controlled atmosphere and allowed to condense on a substrate placed in the vapor stream. As the coating material must remain in the vapor state until it reaches the substrate to be coated, various practical limitations are involved. For example, the distance the vapor can travel before condensation occurs is limited, and direction of the vapor stream is diflicult. Moreover, special precautions are frequently necessary to insure the chemical uniformity of the coatings.

According to the present invention, it has been found that a plasma carrier can be used to transport a suitable film forming material and that this process can be used to produce films of excellent adherency and extremely high chemical uniformity, including photoconductive films suitable for use in xerography and in migration imaging systems. Adherency and uniformity are especially advantageous in these two instancesbecause of the strict physical and electrical parameters that are imposed in high quality imaging and recording.

Generally, it is preferable to practice the present invention with a plasma that will react with the coating material and transport it in a metastable form to, or near, the substrate to be coated. Depending upon the particular use of the coated object produced in accordance with the present invention, it may also be necessary to select the plasma from materials that will not undesirably contaminate the coatings. In producing high quality photoconductive films such as for use in xerography, this technique has been found to be especially satisfactory.

3,472,679 Patented Oct. 14, 1969 SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide an improved method and apparatus for producing highly uniform films on substrates which overcome the above noted disadvantages.

It is another object of this invention to provide a method and apparatus for producing highly uniform photoconductive films contained on a substrate.

It is another object of this invention to provide a method of forming improved imaging members for xerography and migration imaging.

It is yet another object of this invention to provide an improved vapor coating technique.

The foregoing objects and others are accomplished in accordance with this invention by providing a method and apparatus for coating a film on a surface which comprises establishing a carrier stream flowing from a source of coating material toward the surface to be coated, in which the carrier stream is made up of a carrier gas and a coating material. For purposes of illustration, in one embodiment of this invention, an external inductively coupled excitation source is used to generate a carrier gas to the plasma state which reacts with a coating material, such as selenium, or other suitable material placed in a chamber. The carrier gas in the plasma state carries the coating material toward a surface to be coated with the plasma being diminished by means of a grounded electrode at some point intermediate the coating material source and the surface to be coated. The vapor stream then contacts the surface to be coated resulting in a uniform film or coating of the coated material formed on the surface or substrate.

This novel method is particularly adapted to forming photoconductive coatings having xerographic utility in which the material to be coated comprises selenium, arsenic, tellurium, antimony, bismuth, thallium, sulphur, and mixtures thereof. It should be understood that these photoconductive materials may optionally contain small amounts of certain additives which modify or enhance their electrical or physical properties. For example, materials such as halogens, which include iodide, chlorine, fluorine, and bromine, may be added either as or in the carrier gas or be inherently contained in the coating materials themselves in order to impart added sensitivity or spectral response.

The reactive carrier gas for the above grouping of materials preferably includes hydrogen and ammonia. It should be understood, however, that other carrier gases may be used. These include gases containing halogens, hydrides of phosphorus, arsenic, selenium, and antimony, and halides of phosphorus, arsenic, selenium antimony, sulphur, and bismuth. The Group IV, V, VI elements present in these gases will also be deposited out during the coating step and will be included in small amounts in the resultant film.

In one embodiment illustrative of the instant invention, hydrogen is introduced at one end of a reaction chamber maintained at reduced pressure by means of a vacuum pump connected to an outlet tube at the opposite end of the "chamber. An external inductively coupled excitation source is used to generate a plasma which reacts with a source of selenium placed in the chamber. The plasma is terminated or effectively diminished at a predetermined point by means of a grounded electrode, for example, strapped to the outside of the reaction chamber. The coating material is found to deposit on an aluminum substrate positioned in the vapor stream beyond the plasma zone.

The advantages of this improved technique and apparatus will become apparent upon consideration of the following disclosure of the invention; especially when 3 taken in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the reaction tube and apparatus for a continuous flow system according to the present invention;

FIG. 2 illustrates a continuous flow system wherein the coating material is injected in vapor form; and

FIG. 3 is another embodiment of coating apparatus.

FIG. 1 schematically illustrates a continuous flow system for producing coatings in accordance with the present invention. Reaction chamber 5, preferably of glass or quartz, includes a vessel 6 having inlet port 7, and end-plate 8 having outlet port 9. The inlet port is connected to carrier gas supply tank 11; the outlet port is connected to pump 12. As illustrated, the apparatus is provided with inlet and outlet valves, represented at 13 and 14, to control the gas flow.

An excitation source is provided to produce a plasma zone within the chamber. In the illustrated embodiment, this comprises a water-cooled copper coil 15, surrounding part of the chamber and connected to electrical power generator 16. Grounded electrode 17 is provided to terminate the plasma at a predetermined point.

A vapor stream flowing in the general direction indicated by the arrow is established by means of the illustrated apparatus to transport coating material at source 18 to substrate 19 (of a metal such as stainless steel, aluminum, brass, glass, plastic, or the like) located downstream beyond the point where the plasma is terminated.

In operation, carrier gas is continuously fed into the reaction chamber, excited to the plasma state, and then returned to the ground state at a predetermined point downstream. In carrying out the invention, the carrier gas and coating material are used in proper combination so that the plasma reacts with the coating material placed in the reaction chamber and is capable of transporting it towards the member to be coated. It is desirable, however, that the coating material be transported in a sufficiently unstable form so that it will deposit on surfaces (including those of the substrate to be coated) with which it comes in contact beyond the plasma zone. That is, the coating material should be transportable in the plasma phase of the carrier gas but should be sufiiciently unstable in the vapor phase to return to solid form on the substrate to be coated.

In terms of one specific embodiment of this invention as illustrated by FIG. 1; pump 12 is operated to pass hydrogen from the supply tank 11 at a rate of about 1 cc./rnin. (measured at room temperature and atmospheric pressure) through reaction chamber under a reduced pressure in the range of 20-100 microns of mercury. A 100 watt electrical power generator 16, such as a Viking Challenger, Model 240-182 (E. F. Johnson Co., Waseca, Minn.), is operated to supply a 30 megacycle alternating current to coil 15. It is noted that the light intensity emitted by the plasma appears to be highest at the region close to the coil and gradually diminishes in intensity downstream.

In this embodiment, source 18 comprises substantially pure selenium (99.999% pure obtained from the American Smelting and Refining Co.) in the form of pellets to Mt inch in diameter are placed in the plasma zone of the chamber. Deposition of selenium on substrates 19, in the form of a highly uniform film, takes place only on the surfaces beyond the plasma zone; selenium does not coat out within the plasma region itself. Under the above conditions, the thickness of the selenium film builds up at the rate of about 1 micron per hour. It should be understood, however, that the rate of evaporation for any of the aforementioned coating materials may be increased or decreased by simply varying the process parameters by techniques well known to the art.

The operating pressure of the above-described process may be selected within the range of from about 10 microns to several millimeters of mercury, depending upon the power supplied to the coil. Other suitable plasma, such as ammonia, may also be used. The applied alternating current is desirably within radio frequency range.

Examination under an electron microscope of a very thin film deposited as above shows that the film is made up of closely packed nodules approximately 1,000 Angstroms in diameter. Electron diffraction measurements of selenium films deposited by the above-described plasma transport process shown them to be of the amorphous form. Generally to form an amorphous layer of selenium or the other related coating materials and alloys listed above, the substrate temperature should be held at about room temperature. A substrate temperature range of about 20-100 C. has been shown to be satisfactory. If the film is not to be in the amorphous form, temperatures outside this range may be used. Accordingly, the present invention is particularly adaptable for use in imaging by means of migration imaging techniques, as well as other applications employing a thin photoconductive film where high uniformity is desirable. Another application of the instant invention would be the formation of thin films or layers over interfaces, binder layers, and photosensitive or semiconducting surfaces or substrates.

The following example of the application of the instant coating process to a migration imaging system is olfered to further describe the scope of the invention.

The imaging member, or plate, use in migration imaging as more fully described in patent application Ser. No. 460,377, filed June 1, 1965, comprises a discontinuous easily fracturable photoconductive layer overlying a softenable layer coated on a stable mechanical support. Thus, the above-described process may be used to form a thin selenium film on a substrate 19 comprising a 2 micron layer of Staybelite Ester 10 (Hercules Powder Company) on a Mylar polyester film (E. I. du Pont de Nemours Co., Inc.) having a thin transparent aluminum coating. The deposited selenium film is preferably about 0.2 micron in thickness. The plate is then electrostatically charged in darkness to a positive potential of about 60 volts by means of a corona discharge device in accordance with well-known xerographic techniques. Charging is followed by an optical image exposure of 1.5l 10 photons/cm. by means of a 4,000 Angstrom unit light source. The plate is immersed in a bath of liquid cyclohexane for about 2 seconds and removed. A faithful replica of the optical image is thereby produced.

The present invention is not restricted to a reaction of the plasma with coating material in solid form only. As illustrated in FIG. 2, the coating material may be injected into the plasma in vapor form. The apparatus of FIG. 2 is the same as that of FIG. 1, except that the reaction chamber, designated 5', is provided with one or more sidearms represented at 21, adapted to hold a coating material. Each sidearm is provided with a heating means, such as an electric heating element shown at 20. As in FIG. 1, pump 12 is connected to outlet port 9 and tank 11' is connected to inlet port 7'. The apparatus includes valves 13' and 14' and a plasma generating means including coil 15 connected to electrical power generator 16'. Electrically grounded electrode 17' surrounds vessel 6 downstream from the plasma zone. End-plate 6' permits access to the interior of vessel 6' for positioning substrates 19' and the coating material source. In this embodiment, the rate of flow of the carrier gas and coating material in the vapor form should preferably be in a molar ratio of 1 or more in favor of the carrier gas. For example, for every mole of hydrogen, less than 1 atom equivalent of selenium should be used. Ratios of less than 1 can be used, but the above ratio is preferred.

In operation, heat is applied to coating material source 18' causing it to evaporate rapidly whereby it is injected into the plasma zone along with the carrier gas from tank 11'. The illustrated apparatus and components are otherwise operated as described in connection with FIG. 1. It is noted that the rate of deposition on the substrate surfaces represented at 19' is substantially higher in this embodiment of the invention as compared with the embodiment involving reaction of the plasma with solid coating material placed in the plasma zone. The films produced have the same properties in both instances.

FIG. 3 schematically illustrates another embodiment of the present invention. Bell-jar 22 rests on support base 23 which is provided with inlet port 24 and outlet port 25 for connection to the carrier gas supply and an exhaust pump or other evacuation means, respectively, not shown in this figure. The apparatus also includes one or more pedestals represented at 26, each of which is provided with a heater represented at 27. Coil 28 surrounding insulating chimney 29, of glass, is adapted for example, for connection to a radio frequency power source similar to those described above in FIGS. 1 and 2. Substrate 31, supported by any conventional means, is positioned above the chimney in the vapor stream produced when coating material 32 is heated. In FIG. 3, substrate 31 is schematically shown as suspended over the chimney by means of glass hangers 33 attached to chamber 22 by means of suction fittings 34.

Operation of the apparatus illustrated in FIG. 3 is essentially the same as described in connection with FIG. 2. An appropriate carrier gas is introduced into the bell-jar through the inlet port 24, a reduced pressure is created by the evacuation means attached to port 25. A radio frequency current applied to the coil excites the carrier gas in the chimney to the plasma state. The heated coating material which is evaporated upwards is injected into the plasma zone and transported in the direction of the substrate on which it forms as a highly uniform adherent film.

Without intending to limit the present invention by proposing a theory of operation for the plasma transport process described herein, it is surmised that when, for example hydrogen is used that active hydrogen species, such as atomic hydrogen and energetic molecular hydrogen, are formed by electron bombardment of the hydrogen molecule and that these active hydrogen species, upon collision with the coating material, form metastable gaseous hydrides. Presumably, the hydrides decompose on the surface of the reaction chamber and would remain there in the form of a metallic film but for the high intensity of the plasma in the plasma zone. The continued formation of the metastable gaseous hydrides in these regions results in a net deposition of zero. As the glow intensity diminishes, the consequent reduced removal results in a deposition of a coating material. Thus, a film of coating material deposits on surfaces of members placed in the vapor stream.

As stated above, a proper combination of carrier gas and coating material should be used. In this regard, it has been found that certain gases such as argon are ineffective in the described process for transporting selenium, arsenic, tellurium, antimony, bismuth, thallium, sulphur, and mixtures thereof. It is also noted that halogen plasmas such as, for example, chlorine plasma and iodine plasma can be used effectively to produce uniform coatings of the above coating materials. It should be pointed out, however, that when Group IV, V, VI, VIII elements are present in the carrier gas, these elements will also be deposited out in the coating or film. Therefore, the combination of carrier gas and coating material should be selected so as to yield a coating having the desired composition and properties.

Although specific components and proportions have been stated in the above description of the preferred embodiments of this invention, other suitable materials and procedures such as these listed above may be listed with similar results. In addition, other materials may be added to the carrier gas or coating material which synergize, enhance or otherwise modify the resultant film.

What is claimed is:

1. A process for coating a film on a surface which comprises; establishing a carrier gas stream flowing from a source of coating material toward a surface to be coated with said material; said carrier gas stream being made up substantially of a carrier gas and a coating material, exciting said carrier gas into plasma state in the vicinity of said coating material, and diminishing said plasma at a point intermediate said source and said surface to be coated, whereby said carrier gas stream in a vapor state contacts said surface, and a film of said coating material is formed thereon, with said coating material being selected from the group comprising selenium, arsenic, tellurium, antimony, bismuth, thallium, sulphur, and mixtures thereof.

2. The process of claim 1 in which the carrier gas comprises a material selected from the group comprising hydrogen, ammonia, chlorine, and iodine.

3. The process of claim 1 in which the carrier gas comprises hydrogen.

The process of claim 1 in which the carrier gas comprises ammonia.

5. The process of claim 1 in which the carrier gas stream is formed by exciting the carrier gas into the plasma state at the location of the source of coating material.

6. The process of claim 1 in which the carrier gas stream is formed by flowing a mixture of vapors of the coating material and a carrier gas into a plasma forming region and exciting the carrier gas to the plasma state within said region.

7. The process for forming a film on a surface which comprises; establishing a carrier gas stream in the reaction chamber containing a source of coating material, generating a high frequency electrical field to excite a carrier gas to the plasma state in the vicinity of said coating material whereby said material coacts with and is transported in said carrier gas stream, with said coating material comprising a material selected from the group comprising selenium, arsenic, tellurium, antimony, bismuth, thallium, sulfur, and mixtures thereof, and causing said carrier to contact said surface outside the plasma zone produced above, whereby a film of said coating material is formed on said surface.

8. The method of claim 7 in which the carrier gas comprises a material selected from the group comprising hydrogen, ammonia, chlorine and iodine.

9. A process for coating a surface which comprises producing a carrier gas stream flowing through a first zone and a second zone, sequentially, exciting a carrier gas in a first zone to the plasma state, vaporizing a coating material selected from the group comprising selenium, arsenic, tellurium, antimony, bismuth, thallium, sulfur, and mixtures thereof, and injecting the vapor of said material into the carrier gas in said first zone, and terminating the plasma in said second zone whereby said coating material deposits on said surface.

10. The method of claim 9 in which the carrier gas comprises a material selected from the group comprising hydrogen, ammonia, chlorine and iodine.

11. The method of claim 9 in which the carrier gas comprises hydrogen and the coating material comprises selenium.

12. The method of claim 9 in which the carrier gas comprises ammonia and the coating material comprises selenium.

13. The method of claim 10 in which the coating material comprises selenium.

14. The method of claim 10 in which the coating material comprises an alloy of selenium.

15. Coating apparatus comprising:

a reaction chamber in which members to be coated can be placed, said chamber including an inlet port,

an outlet port, and

means to hold a supply of coating material in solid form;

means to establish a carrier gas stream within the reaction chamber, the means including a supply tank for said carrier gas connected to the inlet port,

a pump connected to said outlet port, and

valve means connected to the inlet and outlet ports to control the flow of carrier gas;

means to establish a plasma zone in said chamber, the

means including a coiled electrode surrounding a portion of the reaction chamber,

an electrical power source connected to said coiled electrode, and

a grounded electrode surrounding a portion of the reaction chamber downstream from said coiled electrode; and

means to vaporize the coating material and inject the vapor into the carrier gas stream upstream from the plasma zone.

References Cited UNITED STATES PATENTS 2,501,563 3/1950 Colbert et al. 11793.1 X 3,024,761 3/1962 Bertelsen 118-49.1 3,108,900 10/1963 Papp 117-118 X 3,275,412 9/1966 Skrivan 23-14O X 3,290,567 12/1966 Gowen 117-107 X 3,296,115 1/1967 Laegeid et al. 313-231 X 3,297,465 1/1967 Connell et al. 117--106 X 3,310,424 3/1967 Wehner et al. 117-93.l X 3,329,601 7/1967 Mattox 11793.1 X 3,355,371 11/1967 Hile et al 11793.1 X

ANDREW G. GOLIAN, Primary Examiner US. Cl. X.R. 

